This invention relates to an electronic circuit for modulating a musical tone signal, and more particularly to such a circuit in which the musical tone signal is modulated to produce a rotating sound effect.
The addition of pulsato, tremelo, chorus or other low frequency modulation effects to a musical tone signal enhances the richness of the resultant sounds. Pulsato may be produced using rotary sound channels, as shown in Leslie U.S. Pat. Nos. Re. 23,323, 3,080,786 and 3,174,579 among others. While this technique for producing pulsato has enjoyed wide and long-term acceptance, many investigators have attempted to electronically simulate the desirable effect in order to eliminate the bulk and cost of the rotary speaker, and the attendant mechanical problems.
One such electronic system is known from U.S. Pat. No. 4,008,641 which has three channels each coupled to a respective loudspeaker and each having an amplitude modulator therein. The musical tone signal to be modulated is applied directly to the amplitude modulator in one of the channels and through a delay circuit to the amplitude modulator in each of the other two channels. A frequency modulator is coupled to the amplitude modulator in the first channel and to the delay circuit for frequency modulating the musical tone signals therein, and phase shifters are coupled between the frequency modulator and the respective amplitude modulators in the second and third channels for shifting the phase of the musical tone signal in the channels. The outputs of the amplitude modulators are acoustically reproduced, with the musical tone signal from the first channel being in the center of the reproduced sound and the musical tone signals from the other channels being on opposite sides of the musical tone signal from the first channel. The sound emanating from the center speaker is loudest at the transition between sharp and flat of the frequency modulated signal, and one of the side speakers is loudest when the frequency modulated signal is going from flat to sharp while the other side speaker is loudest when the FM signal is going from sharp to flat; this produces the effect or rotation, but it does not accurately simulate the acoustic effect produced by a rotary speaker. That is, when the FM modulated signal is going sharp, the signal produced by one of the side speakers is more dominant than it should be. Moreover, proper operation of the circuit is highly dependent on the relative placement of the loudspeakers, and also requires rather specific positioning of the listener with respect to the loudspeakers for him to perceive a rotating sound effect.
A device requiring only two loudspeakers for electronically simulating the radiation effects produced by a rotary speaker is known from U.S. Pat. No. 4,162,372, in which circuit the input tone signal is frequency modulated at a sub-audio rate and the frequency modulated signal and the original signal are mixed and applied to two variable gain amplifiers, the outputs of which are applied to respective loudspeakers. The gains of the amplifiers are varied in phase opposition at the aforementioned sub-audio frequency, the modulating signal being applied to the amplifiers through a low-pass filter having a crossover at about 1.0 Hz, so that the amplitude modulation is more pronounced at 0.7 Hz. than at 7 Hz. This has the effect of quite closely simulating the effect in a rotary speaker pulsato generator that amplitude modulation is less distinct in the "fast" mode than in the "slow" mode, but because the amplitude modulation occurs in both channels in synchronism, the system does not simulate the effect of a rotary speaker facing away from the listener. This deficiency of the '372 system is not found in the above-described system, which does simulate the sound that is produced by a rotary speaker when the speaker is facing the back of the cabinet.
Thus, these two known systems, while each simulating to a degree many of the characteristics of the sound produced when a rotary speaker is used to modulate a musical tone signal, fails to simulate other effects, with the consequence that neither accurately simulates the pulsato and radiation effects produced by a rotary loudspeaker. Moreover, the system described in U.S. Pat. No. 4,008,641 is relatively expensive to manufacture and, has been noted previously, requires a particular placement of the loudspeakers relative to each other, and rather specific positioning of the listener with respect to the speakers, to produce the desired results.
Another system for electronically simulating the radiation effects produced by a rotary loudspeaker is described in commonly assigned application Ser. No. 107,203 filed concurrently herewith by Robert A. Finch. The system has two loudspeakers and a musical tone signal to be modulated is applied to a variable delay device associated with one of the speakers. The frequency modulated signal produced by the variable delay device is also amplitude modulated by the same sub-audio modulating signal that controls the variable delay device, and the resulting composite signal is applied as one input to a summing amplifier coupling the signal to the other speaker. The frequency components of the frequency modulated signal above a predetermined frequency are applied as a second input of the summing amplifier and summed with the composite signal, in out-of-phase relationship, to produce an effect simulative of a rotating high frequency horn radiator.
It is the object of this invention to provide a circuit for electronically modulating a musical tone signal by which an adequate rotating sound effect can be produced with minimal cost.